Frame by frame or packet based data transmission is used in a wide variety of protocols. IEEE 802.15.4 defines a standard for wireless transmission with a data transmission rate of up to 250 kbps using a Direct Sequence Spread Spectrum (DSSS) modulation wherein a variety of carrier frequencies can be used in the physical layer. The most widely used variant of the standard for example uses the 2.4 GHz band for transmission with a fixed data rate of 250 kb/s. However, the fixed data rate may not be appropriate for all applications. Beside the standard compliant operating mode, a variety of manufacturers thus provide for proprietary modes that allow for additional speeds of 125 kbps, 500 kbps, 1 Mbps and 2 Mbps or a subset thereof.
With mobile devices battery life and consequently power consumption is an important consideration. In a wireless transceiver the radio when transmitting draws much higher power than the rest of the system in any of the other operational states. The radio transmitter thus is generally kept in a deep power-down state and switched on only for the time it takes to transmit the data. In multi-rate wireless transceivers, the power required for transmitting a data frame is proportional to the time it takes to put it on the air and independent of the rate of transmission. It then follows that the highest rate, permissible by the external limiting factors (system configuration, application requirement, channel characteristics etc.) is to be selected for each individual transmit operation. If, however, the decision is made independently by the transmitting device then the situation, illustrated by the simple wireless network configuration shown on FIG. 1, arises: a receiving device 110 has to learn the transmit rate from the received frame itself. As shown in FIG. 1, the first transmitter 120 and second transmitter 130 each constitute low power nodes which use the highest available data rate allowed by its link quality to save transmit power. The coordinator or receiver 110 needs to synchronize itself without advance knowledge of the incoming frame data rate.
In the universally adopted solution the initial part of a frame is always modulated at the lowest available rate and rate switching occurs only immediately after this. Referring to FIG. 2, again a transmitter 210 is sending a wireless transmission 220, 230 to a receiver 240. The rate switching happens in the Header part 230 when the receiver 240 is already decoding the frame 220. In wireless networks, for instance the ones built on the IEEE 802.15.4 standard, in which the overwhelming majority of data packets typically carry a few tens of byte payloads this leads to gross inefficiency as the comparisons on FIG. 3 indicate.